How to make hair particles (grass) bend on z axis
I'm not sure I understand the question completely, but I'll start with some possible solutions:

Hair Dynamics:

Using hair dynamics will simulate the strands with cloth physics,

Some examples with a wind force:
With hair dynamics:


Without hair dynamics:


If you really want to simulate it (Note, simulation probably takes longer and you can most likely do it satisfactorily other ways) you can use a Smoke simulation to get the airflow infromation and then use a Smoke Flow force to use it to effect the grass:



(The advantage to this is that you get all the complex little air currents caused by the cube as the air flows around it to effect the grass)

Texture Force field

I'm not sure what you mean by not bending (Is it not bending in the viewport? or only when rendered?)

Texture force with Dynamics:


And without: